Radio cellular telephones are known in which a transmitter/receiver (transceiver) is provided with its own local nickel cadmium (nicad) battery. The nicad battery delivers 12V to the transceiver, such that said transceiver may be powered directly from an automobile battery. Thus, such transceivers may be used in an automobile, deriving power via a cigar lighter adaptor for example or, alternatively, they may be used externally from the automobile, deriving power from their own local power source.
With telephones arranged to operate from power supplies of substantially less than 12V, it is possible to power the telephone from a cigar lighter adaptor via a suitable voltage dropping circuit and to simultaneously re-charge the telephone's local battery power pack. Such a system is disclosed in European Patent Publication 0 393 653.
However, transceivers designed to provide optimum performance when used in vehicles are designed to operate directly from a vehicle's battery supply, usually providing 12V. Such systems can be provided with a local 12V battery supply, so that they may be removed from the vehicle and used while disconnected from the vehicle's power supply. Such batteries may also be charged from mains supplies, using suitable transforming and rectifying equipment, however, a problem arises if an attempt is made to charge a telephone's local battery using power derived from a vehicle's battery, via, for example, a cigar lighter adaptor, given that both batteries nominally supply 12V. Thus, a direct connection between the vehicle's battery and the transceiver's battery will only result in the transceiver's battery being charged when it is in a very low-charge condition. As soon as the charge builds up within the transceiver's battery, the potentials of the two batteries will balance and charging current will no longer flow.
Inductive DC/DC convertors are known, in which a DC voltage supply is stepped up using an inductive coil so as to provide a charging current at a higher voltage than the electromotive force on the battery being charged. However, a problem arises if a user wishes to charge a transceiver battery from a vehicle supply while at the same time operating the cellular radio telephone. The inductive nature of the DC/DC convertor, operating at substantially high frequency, will result in radiation which will readily interfere with the operation of the cellular telephone. Such radiation will also tend to interfere with other electronic equipment provided within the vehicle, such as electronic ignition and vehicle entertainment systems, but interference with the cellular radio telephone is particularly acute, given its operating frequency. Thus, when inductive DC/DC convertors are provided it is essential to maintain a significant level of filtering and screening, so as to ensure that the operation of the convertor does not interfere with the operation of the radio telephone. Such filtering and screening significantly adds to the cost of the device, therefore the converting device becomes relatively expensive compared to the cost of the overall system. It is an object of the present invention to provide an improved voltage modifying device.